The invention concerns generally the technology of transmitting and receiving radio signals in a situation where the transmitting and receiving stations are in motion relative to each other. Especially the invention concerns the methods and arrangements which are employed to reduce the adverse effects of doppler shift.
If the transmitting and receiving antennas through which a radio connection is conducted are moving in relation to each other, the receiver sees a doppler shift in all frequency components of the transmission. As an example we may consider a radio connection between a fixed base station and a mobile station located within a moving vehicle, e.g. a car. The largest doppler shifts are encountered when the car is moving at a high speed along a straight road either directly towards the base station or directly away from it. Complicated doppler effects arise from multipath propagation, since there may be two or more competing propagation paths which give rise to differently valued positive doppler shifts and still others which experience various negative doppler shifts. The doppler shift is constantly changing as the relative movement between the transmitter and receiver changes. The adverse effects caused by doppler shift include signal distortion and difficulties in following the transmission on a certain frequency channel. Doppler shift may cause major problems in wireless transmission if there are several frequency channels close to each other, if the attempted data rate over the connection is very high and/or if there is used a modulation method which is prone to errors caused by frequency distortion.
Conventional ways of fighting the adverse effects of doppler shift may be broadly categorized to either changing the tuning of the transmitter or the receiver in order to cancel the frequency changes caused by the doppler shift, or to processing the received signal in order to reconstruct the transmitted signal despite of the distortions. The first approach requires that either there exists some knowledge about the relative motion of the transmitter and the receiver so that the amount of doppler shift can be calculated and the tuning of the receiver (or transmitter) can be changed, or the receiver is able to scan through a frequency band around the nominal reception frequency in order to find the doppler shifted transmission. This approach is not suited for the reception of multiple differently shifted signal components. The latter approach is usually based on the correlational characteristics of digital signals so that a wideband receiver (or a number of parallel narrowband receivers) is used to receive the whole frequency band which includes the most important multipath components, and a digital signal processor works hard to collect the signal energy related to the desired signal and to suppress noise and spurious transmissions.
The above-mentioned conventional methods require carefully designed and complex doppler compensation circuitry in the receiver, which increases the manufacturing costs. Even then the constantly changing arbitrary nature of doppler effects makes it very difficult to set up and maintain high data rate connections e.g. to and from a moving car. As an example we may consider the mobile reception of the known DVB-T (Digital Video Broadcastingxe2x80x94Terrestrial) signals. There have been defined two transmission standards within DVB-T, known as the 2 k and 8 k standards. Both are based on OFDM or Orthogonal Frequency Division Multiplexing, where the bits of a digital transmission signal are distributed onto a number of parallel orthogonal carriers. The 2 k standard involves 2048 orthogonal carriers with the mutual spacing of 4464 Hz and the 8 k standard involves 8192 carriers at a spacing of 1116 Hz. Simulation and practical experiments have shown that at the priority date of this patent application mobile reception of an 8 k signal is only possible when the vehicle speed remains under 80 km/h. The greater spacing between carriers in the 2 k system makes it easier to track and receive correctly even on a moving platform, so mobile reception of a 2 k signal should be possible at speeds up to 300 km/h.
It is an object of the invention to provide an arrangement for reducing the adverse effects of doppler shift in mobile radio communications.
The objects of the invention are achieved by moving an antenna in relation to a moving vehicle so that the relative movement therebetween decreases the relative movement between the antenna and another communicating radio station.
The arrangement according to the invention is suitable for use in a vehicle and comprises an antenna element and means for moving the antenna element in relation to the vehicle. It is characterized in that the means for moving the antenna element are arranged to move the antenna element in relation to the vehicle into a direction which is opposite to a direction of movement of the vehicle.
Above we have noted that although it is typical to name the relative movement of a transmitter and a receiver as the cause of doppler effects, the actual cause is the relative movement between the transmitting and receiving antennas, which is not the same thing. Conventional vehicular antennas are fixed to the vehicle chassis, which causes them to move exactly at the same speed as the vehicle itself. According to the invention there is provided a movable mounting arrangement for a vehicular antenna. When the momentary velocity vector of the vehicle in relation to another radio station is at least approximately known, the movable antenna arrangement is used to generate a corresponding momentary velocity vector between the vehicle and the antenna so that the directions of the two vectors are essentially opposite. Most advantageously also the absolute values of the two vectors are as close to equal as possible.
The general idea of moving an antenna in relation to a moving vehicle can be practically implemented in a multitude of ways. A first embodiment of the invention is applicable to vehicles running on wheels. All parts of the wheels except the very center of each wheel are in a constant movement in relation to the vehicle body. In the coordinate system of the road or track along which the vehicle is moving the parts of the wheels exhibit a certain periodic motion where their velocity vector comes even close to zero at certain times. Several antenna elements may be placed within the wheel and coupled to a switch which repeatedly reselects that antenna element to be coupled to a receiver which has the smallest velocity vector in the coordinate system of the road.
Another embodiment of the invention involves a rotational arrangement of antenna elements placed somewhere else in the vehicle than wheels. There may be even a completely independent rotational antenna consisting of separately selectable antenna elements and freely placed into the most advantageous part of the vehicle. Various measurement methods may be used to determine the instantaneous velocity vector of the vehicle so that the selection of active antenna element(s) is always made optimally to compensate for the instantaneous vehicular velocity.